I have a carter afb that has an unusual choke setup. It has the bi metal spring that opens the choke when heated, but it has no provision for a heat riser and its not electric. It does have a vacuum tap. I thought maybe they ran the vacuum supply thru a ported vacuum switch and used that to pull the piston down to operate the choke, but thought better of that when I discovered that the tap is a vacuum source. So what is the heat source for this choke? The number is C3 9637 S. I believe that's from a early 60's ford. Trying to figure out how to make this choke work properly with the 302 I want to put is on. I could switch to a electric conversion, just trying to save a bit if possible.

The air source for the hot air chokes usually goes into the housing, not the coil cover. If there are no provisions for hot air, this MIGHT be a hot water setup. I've seen early 1960s cars where there was a clip that held the heater hose against the choke coil to provide heat. Some GMs actually had a water hose connections to the choke coil cover itself.